1. Field of the Invention
The present invention relates to a drive unit for driving a movable element and an image data pickup device equipped with the drive unit
2. Description of Related Art
An image pickup device for taking a picture and acquiring a digital image of the picture is incorporated in a wide range of small instruments such as mobile phones and personal digital assistance (PDA) devices. Through the instrumentality of equipping such a small insent always going with the possessor with an image pickup unit the possessor is allowed to take pictures readily without taking pains to carry on a digital camera or a video camera. The small instrument is equipped with a radio or infrared data communication function in general way and capable of being and is in an advantageous position for transmitting a picture image to other mobile phones or personal computers immediately on the scene, in consequence.
The image pickup device installed in such a small instrument like a mobile phone is considerably small in size as compared with ordinary digital cameras and is greatly bounded by size of constituent elements such s lenses and a charge coupled device (CCD) and a space for the constituent elements. Therefore, the small instrument is too inadequate in light of photographic capability and image quality to help substitute for a digital camera and is restricted to a case where image quality is not essential such as a case where an image is used in substitution for a memorandum or a case where an image is used as a stand-by image of a mobile phone.
Recent development of high density micro CCD chips and high contrast micro lenses have enabled rapid improvement on quality of images that small instruments such as mobile phones and PDA devices provide. In order to enhance the photographic capability that is left as a problem to be solved, it is desired that the small instruments have an automatic focusing function and/or a zooming function which ordinary digital cameras are equipped with standard features.
In respect of digital cameras or video cameras, both automatic focusing and zooming are performed by moving a lens or lenses along an optical axis of the lens system by a DC motor or a stepping motor, or otherwise by means of compression-extension of a piezoelectric element. In cases where these motors are incorporated in the small instruments such as mobile phones, in terms of downsizing the instruments and control accuracy of lens movement it is believed that it is preferred to employ a hollow stepping motor comprising a hollow cylindrical rotor mounted on a lens holding barrel and a stator that surrounds the rotor so as to rotate the rotor with a drive pulse of electric current. The stepping motor, known as a pulse motor, is precisely controlled in rotation by controlling drive pulses of electric current applied to the stator. Therefore, the lenses can be moved to a desired position with high accuracy by the stepping motor mounting the lenses in the hollow cylindrical rotor, so as thereby to perform automatic focusing and zooming precisely. Lens drive mechanisms including a hollow stepping motor known from, for example, Published Japanese Patent Application Nos.56-147132, 59-109007 are such that the stepping motor moves a lens barrel along an optical axis through a cam mechanism disposed between the lens barrel and the hollow rotor. An alternate lens drive mechanisms such as known from, for example, Unexamined Japanese Patent Publication Nos. 60-415, 60-416 and 60-417 include a stepping motor that rotation of the hollow rotor itself moves a lens barrel along an optical axis. There has further been proposed in, for example, Unexamined Japanese Patent Publication No. 62-195615 a lens drive mechanism including a stepping motor that has hollow rotor equipped with a lens barrel formed as an integral part thereof.
The stepping motor possibly encounters such a step-out that the stepping motor can not comply with a demand for an unreasonable raise in rotational speed resulting from a drop in ambient air temperature in which a digital camera or a video camera is put to use and things like that, and comes to a halt consequentially. Upon an occurrence of a step-out of the stepping motor, it may fail to put the lens in a best focus position due to a failure in detecting a position of the lens on a lens path, with the consequence that the lens may fail to perform focusing and/or zooming operation.